The present invention relates to inkjet printers in which ink droplets are formed and electrically charged and then deviated to strike a print substrate. It concerns a process intended to correct misalignment defects caused by the differences between the real advance of the substrate and its nominal advance, and a printer embodying such a process.
It is known that a pressurized ink jet ejected through a print nozzle can be broken into a series of individual droplets, each droplet being individually charged in a controlled manner. Constant potential electrodes along the path of these individually charged droplets deviate the droplets by a variable amount depending on their charge. If it is not required that a droplet should reach the print substrate, its charge is controlled such that it is deviated to an ink recovery reservoir. The operating principle of this type of ink jet printer is well known, and for example is described in U.S. Pat. No. 4,160,982. As described in this patent and as shown in FIG. 1, this type of printer comprises a reservoir 11 containing electrically conducting ink 10 that is distributed through a distribution duct 13 to a droplets generator 16.
The role of the droplets generator 16 is to form a set of individual droplets starting from the pressurized ink in the distribution duct 13. These individual droplets are electrically charged by means of a charge electrode 20 powered by a voltage generator 21. The charged droplets pass through a space between two deviation electrodes 23, 24 and are deviated by a variable amount depending on their charge. The least deviated or undeviated droplets are directed to an ink recovery reservoir 22, whereas deviated droplets are directed to a substrate 27. The successive droplets in a burst reaching the substrate 27 can thus be deviated to an extreme low position, an extreme high position and any number of intermediate positions, the set of droplets in the burst forming a vertical line with height xcex94X approximately perpendicular to a relative direction of advance between the print head and the substrate. The print head consists of the droplet generator 16, the charge electrode 20, the deviation electrodes 23, 24 and the recovery reservoir 22. In general, this head is enclosed in a casing not shown. The deviation movement applied to the charged droplets by the deviation electrodes 23, 24 is complemented by a movement along a Y axis perpendicular to the X axis, between the print head and the substrate. The time elapsed between the first and last droplets in a burst is very short. The result is that despite continuous movement between the print head and the substrate, it can be assumed that the substrate has not moved with respect to the print head during the time of a burst. Bursts are fired at regular intervals in space. If all droplets in each burst were directed towards the substrate, then a sequence of lines with height xcex94X would be printed. In general, only some droplets in the burst are directed towards the substrate. Under these conditions, the combination of the relative movement of the head and the substrate, and the selection of the droplets in each burst that are directed towards the substrate, is a means of printing any pattern such as that shown in 28 in FIG. 1. If the line that is drawn with the droplets in a burst is in a direction X, the relative movement of the head and the substrate in the plane of the substrate is in a direction Y perpendicular to X. The undeviated droplets are directed to the recovery reservoir along a path Z perpendicular to the x, y plane of the substrate. Printed droplets reach the substrate by following paths slightly deviated from direction Z.
If the relative movement of the head and the substrate takes place continuously along the largest dimensions of the substrate, there will usually be several print heads printing bands parallel to each other. One example of this type of use is shown in FIGS. 1 and 2 in the patent issued to IBM, as number FR 2 198 410.
If the relative movement of the print head and the substrate in the Y direction takes place along the smallest dimension of the substrate, printing is done band by band, with the substrate performing an intermittent advance movement in the X direction after each scanning. The relative movement of the print head and the substrate is called the xe2x80x9cscanning movementxe2x80x9d. The scanning movement is thus composed of a forward and return movement between a first edge of the substrate and a second edge of the substrate. The movement between one edge and the other edge of the substrate is a means of printing a band of height L, or frequently a part of the band of height xcex94X where xcex94X is usually a sub-multiple of L, without stopping. All bands printed in sequence thus form the pattern to be printed on the substrate. Each time that a band or a part of band is printed, the substrate is advanced by the distance between two bands or parts of bands to print the next band or part of band. Printing may be done during the forward movement only, or during the forward and return movements of the print head with respect to the substrate.
When the pattern to be printed is colored, the different shades of colors are the result of ink impacts from nozzles supplied by inks of different colors being superimposed and placed adjacent to each other. The system for relative displacement of the substrate with respect to the print heads is achieved such that a given point on the substrate is presented in sequence under each of the different colored ink jets. Usually, the print system comprises several jets of the same ink operating simultaneously, either by multiple heads being adjacent to each other or by the use of multi-jet heads, or finally by the combination of these two types of heads in order to achieve high print speeds. In this case, each ink jet prints a limited part of the substrate. The droplets may be produced continuously as described above in relation to FIG. 1. They may also be produced xe2x80x9con demandxe2x80x9d, in other words only when they are necessary for printing needs. In this case, a system for recovery of unused ink is not necessary. Known means of controlling the different jets will now be described with reference to FIG. 2.
The pattern to be printed is described by a numeric file. This file may be formed using a scanner, a calculator aided graphic creation pallet (CAD) transmitted using a calculator data exchange network, or it may simply be read from a peripheral reading a numeric data storage medium (optical disk, CD-ROM). The numeric file representing the colored pattern to be printed is firstly split into several binary patterns (or bitmaps) for each ink. Note that the case of the binary pattern is a non-limitative example; in some printers, the pattern to be printed is of the xe2x80x9ccontonexe2x80x9d type, in other words each position may be printed by a variable number of droplets from 1 to M. Part of the binary pattern is extracted from the file for each jet corresponding to the width of the band that will be printed. FIG. 2, which shows the control electronics of a jet, shows a memory 1 in which the numeric pattern cut into bands is stored, this storage memory containing information about a color. For printing each band, an intermediate memory 2 contains the data necessary for printing the band with the said color. Descriptive data for the band to be printed are then input into a calculator 3 that calculates the charge voltages of the different drops that will form the band for this color. These data are input into the calculator in the form of a sequence of frame descriptions that, when combined, will form the band. The calculator 3 that calculates droplet charge voltages is often in the form of a dedicated integrated circuit. This calculator 3 calculates the sequence of voltages to be applied to the charge electrodes 20, in real time, in order to print a given frame defined by its frame description, as loaded from the intermediate memory 2. An output side electronic circuit 4, called the xe2x80x9cdroplet charge sequencerxe2x80x9d, synchronizes the charge voltages firstly with the times at which droplets are formed, and secondly with the relative advance of the print head and the substrate. The advance of the substrate with respect to the print head is materialized by a frame clock 5, the signal of which is derived from the signal from an incremental encoder of the position of the print unit relative to the substrate. The droplet charge sequencer 4 also receives a signal from a droplet clock 6. This droplet clock is synchronous with the droplet generator control signal 16. It is used to define transition instants of the various charge voltages applied to droplets to differentiate their paths. Numeric data originating from the droplets charge sequencer 4 are converted into an analog value by a digital analog converter 8. This converter outputs a low voltage level and usually requires the presence of a high voltage amplifier 21 that will power the charge electrodes 20. The illustrations of prior art given with reference to FIGS. 1 and 2 are intended to make the domain and benefits of the invention clear, but obviously prior art is not limited to the descriptions made with reference to these Figures. Other arrangements of electrodes and recovery reservoirs for unused ink droplets are described in a very extensive literature. An electromechanical arrangement of charge electrode print nozzles and deviation electrodes as described in invention patent number FR 2 198 410 issued to International Business Machine Corporation (IBM) with reference to FIGS. 1 to 3 in this patent could very well be used in this invention. Similarly, the electronic control circuit for the charge electrodes could be illustrated by the circuit described with relation to FIG. 4 in the same patent. Also, data to be printed need not necessarily be in the form of binary files, but they could be in the form of files containing words of several bits, to translate the fact that each position of the substrate may receive several ink droplets of the same color. It can be understood that for printing, and particularly for color printing, the necessary superposition of droplets originating from different nozzles outputting the different ink colors must be very precise. The main print defects that are generated by all known print systems are related to misalignments along the direction of the relative movement between the print head and the substrate. This defect appears as light or dark lines produced when printing in successive scans. These defects may appear in the space between two bands that in principle must be equal to the interval between two adjacent droplets in a single frame, or within a single band, in the space delimiting the areas printed by different jets, or even inside the frame printed by a jet at the space between two adjacent droplets in the frame. These misalignment defects may be caused either by defects specific to some jets in the print head (mechanical or electrical defects) or substrate positioning errors, or errors of the relative positioning between different print heads, or even between jets in the same print head. Various solutions have been proposed to limit or to eliminate misalignment problems, but all these solutions limit the print rate to a value below the nominal print rate, sometimes by a very high factor, or by redundant print heads and therefore at high cost. Some examples of frequently used known solutions for limiting misalignment will be described very briefly below; a first type of solution is based on fine mechanical adjustments of the positions of print heads by means of micrometric tables. This solution is expensive due to the necessary number of micrometric tables, and frequently painstaking due to the number of trial and error attempts that are necessary.
Another frequently used type of solution consists of using a very high overlap ratio between adjacent drops, in order to avoid white misalignments. These white misalignments correspond to the lack of coverage of the substrate. Dark misalignments are less easily seen and it is preferred to have a misalignment defect composed of dark lines rather than a misalignment defect composed of white lines. The solution consisting of increasing the overlap ratio between adjacent droplets is efficient to compensate for defects within a single band and to a certain extent misalignment defects between bands, but it has the disadvantage that it requires a very large quantity of ink per unit area of substrate and causes difficulties in drying or deformation of the substrate.
A third type of solution for eliminating misalignment defects on printers operating in scanning consists of printing the substrate partially during each scanning. The substrate is completely covered by increasing the number of times that the substrate is scanned. Printing in several passes in this way uses several strategies for interlacing the positions of droplets from different jets. One example of interlacing even and odd lines is given in patent number U.S. Pat. No. 4,604,631 issued to the RICOH Company. One advantage of this solution, frequently related to a high overlap ratio, is that it enables a substrate drying time, but it reduces the print rate by a factor of between 2 and 16.
The performances of colored graphic print systems are naturally moving towards higher and higher resolutions and rates, consequently there is an increasingly critical need to efficiently limit misalignment problems without making compromises that reduce print rates.
The invention relates to the correction of a misalignment defect called a dynamic translation error xcfx86 due to the substrate advancing not enough or too much between two scans. It relates to printers in which the substrate is advanced step by step after each band has been printed.
According to the invention, a mark will be printed when printing each current band. This mark may consist of a single line printed by one or several droplets that may or may not be in consecutive rows. After the substrate has been advanced to print the next band, the error xcex5x will be determined as the difference between the nominal position and the real position of the mark, corresponding to a difference in the advance of the substrate. This error in the advance of the substrate will be compensated by modifying the charge of the droplets printed in this band. This modification will create a path for each droplet that is different from the nominal path. If the modification to the charge is calculated correctly, this path will intersect the substrate surface at a position offset from the nominal position, in the direction opposite the offset in the advance of the substrate.
Therefore, the invention relates to a process for compensation of a defect in the step by step advance of a print substrate by modifying the arrival position of ink droplets on the substrate, these droplets being electrically charged in a variable and sequential manner using charge electrodes, applying a nominal voltage to each droplet as a function of its row in the frame, the droplets originating from a print head, the paths of the droplets being modifiable by deviation electrodes deviating the droplet to one of N nominal positions, between a first position X1 and a last position XN, and including Nxe2x88x922 intermediate positions, the N positions defining the frame in the form of a straight line segment parallel to an X direction of the substrate, characterized in that:
a current band is printed with a first mark on the substrate,
the substrate is advanced to print the next band,
after the substrate has advanced and before the next band is printed,
an algebraic difference is determined between a nominal theoretical position of the mark and the real position of the mark,
a substrate advance correction is determined for each droplet in the burst, consisting of a dynamic translation correction voltage xcfx86 to be applied to the value of the charge voltage to be applied to each droplet output from the head to correct the deviation of the droplets and compensate for the algebraic difference of the position of the substrate from its nominal position,
the calculated dynamic translation correction voltage xcfx86 of the substrate position is applied to each droplet in the burst directed towards the substrate, in addition to the nominal voltage.
The invention also relates to a printer equipped with means of making the process according to the invention. It is a printer with a continuous deviated jet projecting droplets in rows 1 to N in the burst, the droplets in one burst possibly but not necessarily being directed towards a print substrate as a function of data defining a pattern to be printed, the printer having at least:
a print head, this head comprising means of separating at least one inkjet into droplets and an associated droplet charge electrode, means (23, 24) of deviating some of the droplets towards the print substrate,
print control means, comprising means of injecting the charge to the droplets to be directed towards the substrate as a function of the rows of the droplets in the burst, coupled to the droplet charge electrode,
characterized in that the print control means comprise at least a mark position detector, this detector outputting a representative value of a difference between a nominal advance and a real advance of the substrate and in that the print control means also comprise a calculator for calculating the dynamic translation correction voltage xcfx86 for the substrate advance, this calculator determining a dynamic translation correction voltage xcfx86 for the substrate advance for each droplet in a burst depending on its row, this correction voltage also taking account of a value of the substrate advance error output by means coupled to the detector and calculating values of errors from a nominal position, the calculator calculating the dynamic translation correction voltage xcfx86 for the substrate advance being coupled to droplet charging means, the droplets charging means taking account of the value of the dynamic translation correction voltage xcfx86 for the substrate advance generated by the calculator calculating the dynamic translation correction voltage xcfx86 for the substrate advance to modify the charge voltage of each droplet as a function of the dynamic translation correction voltage xcfx86 for the substrate advance.